1. Field of the Invention
The present invention relates to a switching regulator device, and particularly to a switching regulator device that has a power conversion transformer and supplies power to a load circuit under the state that a primary side and a secondary side are insulated from each other.
2. Description of the Related Art
There has been known a flyback type switching regulator device that has a flyback transformer as an insulation type switching power and supplies power to a load circuit under the state that a primary side and a secondary side are insulated from each other.
This flyback type switching regulator device is controlled so that an output voltage at the secondary side is monitored and feedback control is performed to output a predetermined voltage (for example, see Japanese Patent No. 2,679,581 and Japanese Patent No. 3,399,242).
The following three methods, etc. may be known as a feedback control method.
(1) A method of transmitting an output voltage (voltage information) at the secondary side to the primary side through an insulating element such as a photocoupler or the like, and using the output voltage for feedback.
(2) A method of detecting an output voltage by using a tertiary coil (feedback coil) and using the output voltage for feedback.
(3) A method of estimating an output voltage on the basis of a drain voltage (flyback voltage) of MOSFET to be switched and using the output voltage for feedback.
In the methods described above, according to the method indicated by (1), the output voltage is directly monitored and thus a predetermined output voltage can be output with high precision with respect to load variation at the secondary side. Conversely, in the case of a multi-phase output type switching regulator device, phases out of a phase being monitored cannot be controlled, and thus this method has a problem that cross-regulation occurs.
As compared with the method of (1), the method of (2) has a characteristic that no insulation device is required. However, the load of the feedback phase is constant and thus this method cannot follow load variation at the secondary side. Therefore, this method has a problem that cross-regulation occurs.
The method of (3) basically has the same problem as the method of (2).
The following is a cause of occurrence of the cross-regulation. A feedback phase (feedback system) out of plural output phases in the multi-phase output type switching regulator device is kept constant, and thus the other output phases cannot be controlled. That is, with respect to the multi-phase output type switching regulator device, each of the output phases cannot be individually controlled, and thus when the load condition varies, the output voltage also varies. Accordingly, there is a tendency that the output voltages of the other phases is higher than that of the feedback phase under a low load, and also there is a case where the output voltages are reduced under a high load.
Furthermore, a bleeder resistor is used at the feedback phase so that the same current as the current flowing at the output phase flows at the feedback phase. This bleeder resistor is used to consume power, and thus the power consumption is increased as a whole of the switching regulator device. Therefore, the bleeder resistor is a factor of causing reduction of the effective power conversion efficiency.